The present invention relates to optical recording media having composite dielectric layers and more particularly, to an improved optical recording medium having a composite dielectric layer formed of aluminum nitride and silicon nitride having a specific composition and refractive index.
Information can be written onto prior art optical recording media by irradiation of condensed laser light onto a perpendicular magnetic recording layer in order to invert the magnetization of the recording layer. Alternatively, laser light irradiated onto the recording layer can be used to change the crystalline structure of the layer. Specifically, the crystalline structure can be changed from crystalline to amorphous, from amorphous to crystalline, from hexagonal to cubic, from cubic to hexagonal and the like by making holes, forming bubbles or otherwise transforming the recording medium using laser phase transition.
Such optical recording media generally have plastic substrates or supports including polycarbonate, polymethylmethacrylate and epoxy resin supports with guide grooves formed therein. The plastic supports can be formed by injection molding in order to reduce cost and permit mass production. However, plastic supports are not completely satisfactory because they are susceptible to moisture absorption and are gas permeable. As a result, magneto-optic recording layers containing alloy films of rare earth metals and transition metals such as GdTbFe, TbFeCo, GdTbFeCo, DyFeCo, NdDyFeCo and NdDyFeCoTi are readily attacked by atmospheric moisture and gasses and the magnetic properties deteriorate significantly.
Dielectric oxide layers are provided between the plastic supports and the magneto-optic recording layers thereon in order to improve corrosion resistance and overcome the disadvantages of the synthetic resin supports. The protective effect of such dielectric layers is not completely satisfactory because free oxygen in the dielectric oxide layer tends to oxidize the optical recording layer.
Alternatively, non-oxide dielectric films such as silicon nitride, aluminum nitride and zinc sulfide have been formed on plastic supports. These non-oxide dielectric films do not completely adhere to the support and recording media provided on the dielectric films crack as a result. This method is therefore not suitable for practical use.
Accordingly, it is desirable to provide a composite dielectric layer that overcomes the disadvantages of prior art dielectric layers.